1. Field of the Invention
This invention relates to an optical device in which an optical element is sealed with a glass material, and to a method for making the optical device.
2. Description of the Related Art
Conventionally, optical devices are known in which an optical element such as a light emitting diode (LED) is sealed with a transparent resin material such as epoxy resin. The optical devices have the problem that the transparent resin deteriorates due to a light emitted from the optical element. In particular, when group III nitride compound semiconductor to emit a light with short wavelength is used as the optical element, the transparent resin near the element turns yellow due to a high energy light emitted from the element and a heat generated from the element itself, so that light emission efficiency may be decreased with time.
In order to prevent the deterioration of the sealing material, a light emitting device has been proposed that uses a low-melting glass as a sealing material (e.g., JP-A-08-102553 and JP-A-11-177129).
JP-A-08-102553 discloses a light emitting device that circumferences of the top ends of an LED element, a wire bonding portion, and a lead portion are covered with a transparent sealing material of a low-melting glass. The low-melting glass may be a glass with selenium, thallium, arsenic, sulfur etc. added thereto to yield a melting point of 130° C. to 350° C.
Further, JP-A-11-177129 discloses a light emitting device that a GaN based LED element with a refractive index of about 2.3 is sealed with a low-melting glass with a refractive index of about 2. In this light emitting device, by using the GaN based LED element and the low-melting glass with the refractive index described above, a light to be totally reflected on the interface between the LED element and the low-melting glass can be decreased.
However, the light emitting devices in JP-A-08-102553 and JP-A-11-177129 need to be produced by processing the still hard glass material even when using the low-melting glass, so that it is impossible to realize the devices by using the conventional resin sealing process. Further, properties and components necessary for realizing the device are not known yet.
Therefore, the inventors checked up issues in realizing the inorganic material sealing process and solved them, where a light emitting device is proposed that can actually obtain effects expected by the glass sealing (e.g., JP-A-2006-108621). JP-A-2006-108621 discloses the light emitting device that the substrate and the glass member are bonded to each other by hot pressing, where a ceramic substrate with an LED chip mounted thereon is supported by a lower mold, and a plate glass member is supported by an upper mold.
However, the light emitting device in JP-A-2006-108621 needs to simultaneously satisfy two processing conditions during the hot pressing, where one is to bond the glass member to the ceramic substrate and the other is to separate the glass member from the upper mold (herein, defined as mold separation property). Therefore, in order to satisfy the two conditions, the acceptable range of temperature of the glass member, pressure applied to the glass member etc. during the hot pressing may be limited, so that the processing may be difficult to apply to a certain material for the substrate and the glass member.